1. Field of the Invention
The present invention concerns improvements relating to flammable liquid storage, namely a container for storing flammable liquid and a method of reconfiguring the container to obviate so-called hydrodynamic ram effects within the liquid. More particularly, though not exclusively, the present invention relates to a fuel tank of a military aircraft, aiming to reduce the risk of catastrophic failure of the tank following penetration by a projectile.
2. Discussion of Prior Art
The fuel tank is one of the most vulnerable components within a military aircraft when it is under attack, particularly during low-level flying operations. Penetration of the tank can result in limited loss of fuel at best, but severe disablement or destruction of the aircraft at worst.
If the tank is pierced by an incendiary projectile, the projectile""s burning trail can ignite the fuel load and cause a devastating, rapidly progressive fire. However, penetration alone by projectiles of a sufficiently large diameter can cause catastrophic failure of the tank by way of hydrodynamic ram effects within liquid fuel.
When a projectile impacts the fuel tank it transfers much of its kinetic energy to the liquid held inside. If the projectile goes on to pierce the tank it violently displaces fuel at the point of entry, radiating a shock wave from the impact site. Although the liquid is able to dissipate this wave quickly, the liquid is relatively incompressible and is unable to absorb the pressure waves generated by the projectile as it traverses the tank. Such pressure waves propagate with the sonic velocity of the liquid, which is far greater than that of the projectile. The pressure waves are thus transmitted ahead of the projectile, creating intense pressure at the point where the projectile will impact the internal wall of the tank. These hydrodynamic ram effects within the liquid fuel cause the projectile""s exit aperture from the tank to be far larger than its entrance aperture.
As a result of space ergonomics, fuel tanks in jet aircraft are generally located alongside the engine inlet ducts. The fuel tank can therefore be pierced in such a way so as to severely rupture the wall shared with the engine duct. Large quantities of fuel can then be drawn into the engine of the aircraft, causing an explosion of massive proportions.
In addressing the problem of catastrophic fuel tank failure, both the space occupied by the tank and its weight must be kept to a minimum, whilst the capacity of the tank should be maintained.
Various attempts have been made previously to address fuel tank failure. For example, U.S. Pat. No. 4,345,698 describes a self-sealing tank which is able to seal apertures made by smaller projectiles and maintain the integrity of the tank, as the associated hydrodynamic ram effects are of little consequence. This method of protection is not effective against larger caliber projectiles, such as 23 mm HEI (high explosive incendiary) ammunition, which as well as producing apertures which are too large to be self-sealed, also generate considerable hydrodynamic ram effects within the fuel.
U.S. Pat. No. 4,469,295 describes a segmented fuel tank, where fuel is first removed from an auxiliary tank to form a protective crumple zone at one side of a main tank, in order to attenuate the hydrodynamic ram effects caused by larger projectiles. The required protection is only available once the fuel in the auxiliary tank has been used up and this can present a problem for protection of the tank at an early stage in the aircraft""s mission. Also separate pumps are required for the auxiliary tank and the main tank, which increases the weight of the fuel distribution system.
U.S. Pat. No. 4,886,225 describes a fuel tank with an inflatable bladder attached along the fuel tank wall shared with an engine inlet duct. This inflatable bladder only provides a protective barrier when inflated, usually when the aircraft is under attack. However, the bladder is inflated at a preselected time during flight, usually after a certain quantity of fuel has been consumed. The limited protection afforded by this device is not available initially and so the aircraft can be vulnerable for an initial period of time.
The previous attempts to reduce catastrophic fuel tank failure thus far have had only limited success. The US Air Force, for example, presently cites hydrodynamic ram effect-related deaths as the main source of fatalities on board military aircraft.
It is therefore an objective of this invention to substantially reduce or overcome at least some of the above described problems. More specifically, it is desired to reduce the present vulnerability of aircraft fuel tanks in the face of enemy fire.
It is a further aim to improve on the designs of currently available aircraft fuel tanks.
According to one aspect of the present invention there is provided a container for storing a flammable liquid, the container comprising a first and a second chamber in fluid communication with each other, and displacing means for displacing the liquid from the first chamber to the second chamber on demand for creating a protective barrier within the container.
The present invention provides control in the creation of the protective barrier and this enables the protective barrier to be formed at any time on demand. Furthermore when the displacing means is activated, any fuel present in the first chamber can be transferred into the second chamber to maximise the amount of protected fuel storage available at that instant in time.
Preferably, the second chamber is contained substantially within the first chamber. This provides improved dimensional protection of the second chamber and reduces the possibility of the hydrodynamic ram effect occurring on impact of a projectile. In fact, the liquid retaining walls of the second chamber may together form a cavity surrounding the second chamber such that the displacing means is arranged to create the protective barrier substantially surrounding the second chamber in use. This advantageously provides maximum protection from the effects of hydrodynamic ram caused by projectile penetration because a penetrating projectile from any direction will never encounter a liquid retaining wall. Any projectile would also have to pass through two tank walls before gaining contact with the flammable liquid.
In order to advantageously expedite rapid evacuation of liquid from the first chamber to the second chamber, the fluid communication between the first and the second chambers preferably comprises a plurality of fluid passageways.
The displacing means may be arranged to introduce a gas into the first chamber to effect the displacement of the liquid into the second chamber. The resulting protective layer of gas is able to dissipate the pressure waves generated by the intrusion of the projectile, as it is readily compressible in comparison to the non-flammable liquid. Consequently any exit aperture of the projectile will be comparable in size to its entrance aperture, preventing the sudden escape of a large quantity of fuel. Preferably, the gas is non-flammable thereby extinguishing any projectile""s burning trail and, more preferably, the gas is substantially inert (like, for example, nitrogen or carbon dioxide), providing no storage hazard.
The container may further comprise collecting means for collecting the gas from the second chamber and recycling the gas for use with the displacing means. In this way, the amount of gas required to maintain the protective barrier is minimised, thereby improving space and weight characteristics over prior art systems.
The container preferably comprises a fuel tank for use in an aircraft. The advantages of the present invention are best realised in the context of such an aircraft fuel tank. However, it would be possible for the present invention to be applied to various military fuel transportersxe2x80x94such as ships and vehicles where there is a risk of projectile penetration under enemy fire.
The present invention also extends to a reconfigurable fuel storage system comprising a container, as described above, and control means connected to the displacing means of the container for controlling the activation of the displacing means. In addition, an aircraft employing such a reconfigurable fuel storage system or a container described above, is also part of the present invention.
According to another aspect of the present invention, there is provided a method of reconfiguring a container for storing a flammable liquid, the method comprising providing the flammable liquid to a first and a second chamber of the container, having fluid communication therebetween, and, on demand, creating a protective barrier within the container by displacing the liquid from the first chamber to the second chamber.